The world's interest in restrictions on emissions of green house gases such as methane (CH4) and CO2 has been increasing over the years. Methane, in particular, is 21 times more potent than CO2 in driving the global warming, and therefore methane emissions to atmosphere must not be left unnoticed. In the meantime, a large amount of methane, as much as 10 to 40 Nm3 (pure methane) per ton of coal, is being released during coal mining from a coal mine.
Coal mines emit two types of methane gas: CMM (Coal Mine Methane) gas (with a concentration of about 20 to 50 wt %) existing in and recovered from coal seams through degassing bore holes using vacuum pumps for safety reasons, and VAM (Ventilation Air Methane) gas (with a concentration of less than 1 wt %) released through venting from the mine tunnels and the coal face.
Therefore, effective use of the methane contained in the gases emitted from coal mines by capturing it before it is released to atmosphere will make significant economic and social contributions.
Patent Document 1 discloses a gas turbine capable of making use of gases with a methane concentration of below the explosion limit, such as landfill gas produced in the landfill, or the gases emitted from coal mines as noted above, as fuel.
Patent Document 2 discloses a gas engine for power generation using methane gas emitted from a coal mine as fuel. The gas engine power generating facility disclosed in Patent Document 2 will be described below with reference to FIG. 7. FIG. 7 is a schematic illustration of a coal mine CM and a gas engine power generating facility 200 built near the coal mine CM. In FIG. 7, inside the coal mine CM are layers of coal seams C0 and the coal seam being mined C1. A ventilation hole 206 is provided for communicating the inside of the coal mine with the outside.
In the coal face 204 of the coal mine, degassing bore holes 208 are drilled in the coal seam being mined C1, and CMM gas emitted from the bore holes 208 is sent to the gas engine power generating facility 200 through a pipe 210 disposed inside the ventilation hole 206 by means of a vacuum pump 211. VAM gas b emitted from the coal mine through the ventilation hole 206 is sent to the gas engine power generating facility 200 through a pipe 212. Electric power E and steam S generated through operation of the gas engine power generating facility 200 are sent to a utility facility 202 in the mine premises or further to other consumers.    Patent Document 1: Japanese Patent Application Publication No. 2010-19247    Patent Document 2: U.S. Patent Application Publication No. 2005/0205022
Gas engines that use methane gas as fuel are an internal combustion engine expected to be used widely in future because of their advantage that they cause very little environmental pollution. However, the amount of methane emissions from a coal mine varies largely depending on the time, and therefore how to maintain a stable amount of supply to the gas engine is an issue to be addressed.
The air-fuel mixture ratio, or an excess air ratio, needs to be maintained at an optimal level in order to reduce the concentration of NOX in the exhaust gas or for other reasons. Maintaining a predetermined excess air ratio, however, is not easy because of the large variations in the amount of methane emissions from a coal mine as mentioned above.
In a gas engine, the temperature of gas mixture supplied to the combustion chamber has to be kept in a constant range of from 40 to 45° C., as otherwise there is a possibility of abnormal combustion such as knock or the like. For this reason, the intake gas having higher pressure and temperature after passing through a turbocharger is kept in a constant temperature range by a charge air cooler (intercooler). With the use of the methane gas emitted from a coal mine, however, it is not easy to control the temperature of the gas mixture supplied to the combustion chamber because of the large variations in the amount of methane gas and performance limitations of the intercooler.